1. Field of the Invention
The present invention relates to refrigerated display cabinets and more particularly to an electrically heated door structure for use in such cabinets.
2. The Prior Art
In U.S. Pat. No. 3,724,129 to Stromquist, the use of an optically transparent electrically conductive coating for preventing moisture condensation on the front-most glazed surface of a refrigerated cabinet door window is described. Such coatings, normally applied to the unexposed surface of the frontmost glass pane are currently in wide use. The amount of heating normally supplied typically ranges from ten to fifteen watts per square foot of window area. Once the door is fabricated, the electrical resistance of the door window heating coating is fixed.
In a typical door structure, the multi-paned window is surrounded by a metallic door frame. To prevent condensation and frost from forming on the door frame, it is known in the art to provide electrical heating elements concealed within a groove within the door frame structure. Normally, once the heating element in the door frame has been installed, its electrical resistance is also fixed.
Theoretically, the heating elements both on the glass and in the door frame could be preselected before the door is built to have heating characteristics suitable for the anticipated environment in which the door will be used. However, as a practical matter, this practice complicates the manufacturing and inventorying of such doors. Further, the use of preselected heating elements of constant resistance normally provides only limited ability to cope with environments which may vary widely.
Controllers are known in the prior art for applying the electrical heating current intermittently, the duty cycle typically depending upon the relative humidity of the ambient air. These controllers do not alter the resistance of the heating elements and as a result, a high resistance door operated in a very humid environment might prove to be inadequate even though the controller maintains the current "on" 100% of the time. In such a case, if the high resistance door is replaced by a lower resistance door, a wasteful overheating of the door may occur.
Prior to the present invention, it has been known to connect the heating element in the door frame to the window heating element in either series or parallel connection depending on the environment in which the door would be operated. Parallel-wired doors were used in high-humidity environments, while series wired doors were used in less demanding environments. Once a door had been built, it was thereafter referred to as a series-wired door or a parallel-wired door, and the interconnection was not thereafter alterable without disassembling the door to a considerable degree.
As a result, it was necessary to inventory both parallelwired doors and series-wired doors both for original installations and for replacement use.
Because doors carried in inventory are unproductive investments, it is desirable to reduce inventory levels to the lowest practical point.